The overall goal of this project is to examine immunological mechanisms which may be involved in the pathogenesis of autoimmune diseases such as multiple sclerosis (MS). Specifically, the antigen specificity of T cells from patients with diseases such as MS are examined and the phenotype of the responding T cells is characterized. Recent studies have resulted in two important observations. First, in distinction to myelin antigens studied previously such as myelin basic protein (MBP), a substantial T cell response has been demonstrated in many patients to a novel myelin antigen, 2, 3-cyclic-nucleotide-3-phosphodiesterase (CNPase). Consequently, the range of reactivity to myelin antigens in MS is extensive. To date, no unique reactivity or unique phenotypic characteristic has been identified for reactive T cells from patients with MS indicating that, if these T cells are part of the disease process, other factors must also contribute to susceptibility. Recent studies of an immunodominant region of MBP indicate that considerable variability exists with respect to the magnitude of the response to the antigen. this finding has now been shown to indicate that many T cells responding to the immunodominant region of MBP can be shown to produce a greater response to an altered antigen suggesting that many autoreactive T cells are derived from reactivity to other antigens, possibly bacterial or viral. More recent examination of this question has employed combinatorial peptide libraries which provides a very powerful tool for dissecting specificity of T cells. Using this approach, peptides differing completely from the peptide used to derive a T cell line have been shown to induce reactivity. Thus, the concepts of molecular mimicry must be redefined and the search for antigens capable of stimulating autoreactive T cells will require new approaches. Events involved in antigen recognition have also been shown to influence the phenotype of the responding T cell. Three distinct mechanisms of T cell-mediated lysis of target cells have been demonstrated in human autoreactive T cells. These include perforin-mediated lysis and Fas-Fas ligand-mediated lysis. A third mechanism involves DD56+myelin reactive T cells that are capable of lysing target calls expressing DC56/NCAM. This mechanism could have particular relevance to diseases of the nervous system.